This is a question that crops up all the time and it can really infuriate experienced photographers (e.g. Karl Taylor).
The answer I prefer to give is “one that takes pictures”*
Yes my answer does have a dose of sarcasm but with good reason. Camera manufacturers slick advertising campaigns have brainwashed the public into thinking that buying a better camera will make them take better pictures. But here’s the truth… THEY WON’T.